The invention relates to a strain of Saccharomyces cerevisae characterized in that it is selected from strain No.I-3936 deposited on March 4th 2008 at the CNCM, strain No.I-3937 deposited on March 4th 2008 at the CNCM, strain No.I-3938 deposited on March 4th 2008 at the CNCM, and strain No.I-3939 deposited on March 4th 2008 at the CNCM. The invention also relates to a phytosanitary composition and to a method for treating or protecting plants against diseases caused by pathogens, using said strain.